A signal processing device that processes a signal input from an input line with an electronic circuit in a low temperature state or a high temperature state to output the signal from an output line, is desired to block heat between the outside and the signal processing device. For example, in a case where a superconducting filter that processes a signal at a low temperature is used as the electronic circuit, there is a risk that the load of a refrigerator increases in order to maintain the low temperature as a heat inflow from the outside increases. In addition, for example, in a case where a power amplifier that processes a signal at a high temperature is used for the electronic circuit, there is a risk that a characteristic of an electronic circuit provided outside degrades as a heat flow to the outside increases.